1. Field of the Invention
The present invention relates to an energy-trapping type piezoelectric resonance device, and more particularly, it relates to an energy-trapping-type piezoelectric resonance device utilizing the mode of thickness-extensional vibration.
2. Description of the Prior Art
U.S. Pat. No. 3,384,768 discloses an energy trapping type piezoelectric-resonator utilizing the mode of thickness extensional vibration, which is prepared by lead zirconate titanate (PZT) piezoelectric ceramics. As shown in present FIG. 2, such an energy-trapping-type piezoelectric-resonator is formed of a piezoelectric ceramic 21 and electrodes 22 and 23 on middle parts of both surfaces thereof. The piezoelectric ceramic 21 is prepared from a material such as PZT piezoelectric ceramic, which has an effective Poisson's ratio of at least 1/3. Utilizing a material having an effective Poisson's ratio of less than 1/3, it is impossible to obtain a frequency-lowering-type of energy trapping piezoelectric-resonance device.
As to the aforementioned piezoelectric-resonator, a demand exists for an improvement for higher frequency use. In order to vibrate such a piezoelectric-resonator in a higher frequency range, the piezoelectric ceramic 21 may be reduced in thickness. However, the lower limit of such thickness is about 200 .mu.m in view of handling, and it is difficult to further reduce the thickness in practice. Thus, an actual frequency range of thickness extensional fundamental vibration has been not more than 10 MHz.
On the other hand, it may be possible to use a piezoelectric-resonator in a higher frequency range by utilizing the third harmonic of thickness-extensional vibration. However, the third harmonic is considerably lower in response level as compared with fundamental vibration. Thus, the third harmonic has been restricted to specific use.
Further, in a conventional piezoelectric resonance device utilizing the mode of thickness-extensional fundamental vibration, it has been impossible to trap energy by using a frequency-lowering method unless the material therefor has an effective Poisson's ratio of at least 1/3, as hereinabove described. Thus, the useful materials has been limited.
As to lead-zirconate-titanate ceramics well known as a piezoelectric material, a composition for enabling energy trapping of thickness-extensional fundamental vibration with an effective Poisson's ratio of at least 1/3 must be near the phase boundary between a tetragonal phase and a rhombohedral phase. Thus, it has been impossible to obtain a device having a low dielectric constant suitable for use in a high frequency range, and the conventional device has been inferior in temperature stability.